1. Field of the Invention
The present invention relates to seed crystal holder providing a space for a seed crystal to rest which will prevent the seed crystal from tipping over and also serves as a pre-heater for the seed crystal. The seed crystal holder is typically used in an apparatus for producing single crystal silicon, and specifically, in an apparatus for producing single crystal silicon in which the single crystal is grown using the FZ (float zone) method.
2. Description of Related Art
The apparatus disclosed in Japanese Patent Application, First Publication No. Hei 7-10681 is a conventionally known example of an apparatus for producing single crystal silicon in which the FZ method is employed. This apparatus for producing single crystal silicon is provided with a housing which has an inert gas environment internally; a polycrystal holder that is installed to an upper drive shaft (positioning rod) inside the housing, and which holds at its bottom end a polycrystalline silicon rod that is the sample material; a seed crystal holder that is installed to a lower drive shaft (positioning rod), and which holds at its top end a seed crystal for the silicon single crystal; and a high-frequency induction heating coil that is provided at a central area inside the housing. In this apparatus for producing single crystal silicon, the polycrystalline silicon rod which is the starting material is held by the polycrystal holder above and the seed crystal for the silicon single crystal is held by the seed crystal holder. With this arrangement in place, the high-frequency induction heating coil is then employed to melt and fuse one end of the polycrystalline silicon to the seed crystal. Thereafter, the polycrystalline silicon rod is rotated relative to the high-frequency induction heating coil and undergoes relative displacement in the axial direction. A single crystal silicon rod is thereby produced by sequential zone melting of the polycrystalline silicon rod in the axial direction.
This apparatus for producing single crystal silicon is provided with a high-frequency induction heating coil for heating the polycrystalline silicon rod. Because the polycrystalline silicon rod has a high specific resistance when it is cold, an exothermic ring (susceptor) comprising carbon, etc., is provided for preheating the polycrystalline silicon in an initial step by radiant heat. First, this exothermic ring undergoes induction heating to reach a high-temperature state. The polycrystalline silicon rod is then heated using this radiant heat. After the temperature of the polycrystalline silicon rod increases to reach a state that enables transmission of electricity, the polycrystalline silicon releases heat on its own as a result of this induction heating. Once this state has been reached, the exothermic ring is receded from around the polycrystalline silicon rod. Thereafter, the polycrystalline silicon rod subject to direct induction heating, and the bottom end thereof melts. This molten bottom end is then fused to the seed crystal, and the single crystal thus grows. The single crystal is releasably retained in the seed crystal holder so when the single crystal is completed, it can be easily separated from the seed crystal holder for further processing.
Known seed crystal holders typically consist of a cylindrical chuck with an open end for receiving a seed crystal and a plurality of set screws for securing, by tightening, the seed crystal in the seed crystal holder. Sometimes an additional ring is added around the portion of the seed crystal protruding from the seed crystal holder as a preheater. Typically both the seed crystal holder and preheating ring are made of tantalum. Heating times for heating the seed crystal using known seed crystal holders and preheating rings are about 5-6 minutes using a high-frequency induction heating coil, also known as a RF coil.